As is known, in the art there exist devices designed to reduce emissions of nitrogen oxide that comprise a catalytic system in which, injected into a compartment traversed by the exhaust gases, is a solution of urea that is to dissociate thermally so as to liberate ammonia, which combines with the nitrogen oxides so as to reduce them and obtain free nitrogen and water. This catalytic system is referred to as selective catalytic reduction (SCR). The system in question carries out a control on the value of the concentration of urea in the solution of urea and water used in order to calibrate its own operating mode appropriately. In particular, as a function of the concentration value, the system varies the flow rate of solution injected into the reaction chamber. Furthermore, the system checks that the concentration value remains within a predetermined range so that proper operation of the system itself is guaranteed.
The foregoing imposes the need to install on the vehicle a sensor device designed to determine the concentration of urea in the solution.
In the technical field in question there is moreover felt the need to be able to verify on board the vehicle that the solution for the catalytic system mentioned above effectively contains urea. There has in fact been noted the risk of other liquids accidentally mixing with the solution in question, such as for example engine coolant or diesel. Coolant may, in the case of leakages in the coolant-circulation system, mix with the solution of water and urea within the reservoir that contains the solution, which is heated by the coolant during starting of the vehicle engine. Diesel, instead, may be introduced by mistake into the reservoir of the solution of urea and water in the cases where this is located adjacent to the diesel tank. Another cause of the presence of foreign liquids is represented by fraudulent actions whereby, instead of the solution of urea and water, liquids of some other type, whether surrogates or not, having a far lower cost are supplied.